Angioplasty, including percutaneous transluminal angioplasty (PTA) or percutaneous transluminal coronary angioplasty (PTCA), is a method for widening or reopening narrowed or obstructed blood vessels (usually arteries, at times also veins). A common method used in angioplasty is balloon dilation.
Balloon dilation within the context of angioplasty is understood in interventional radiology, cardiology, and angiology as a method for dilating pathologically narrowed blood vessels by way of a balloon catheter, a vessel catheter having a balloon attached thereon. A balloon may be inflated slowly under high pressure (e.g., 6-20 bar) only after it has been navigated to the narrowed site. In this way, occlusions created primarily by atherosclerotic changes (sclerosis of the blood vessels) are expanded so that they no longer, or less severely, impair the blood flow.
To this end, the balloon catheters are generally inserted to the site of the stenosis (occlusion) from the groin using a guide wire and guide catheter and are inflated with pressure. In this way, the stenosis is eliminated and surgery is avoided.
Aside from this, catheters comprising a deflatable balloon are also used for the placement of stents. To this end, in the region of the deflatable balloon the catheter carries a stent, which can be placed into the vessel, after the desired site in the blood vessel has been reached, by deflating the balloon.
Modern methods in the field of plastics processing allow such balloons to be designed and continuously developed so as to individually adapt the quality to the needs of the patients. The flexibility of the balloons and the pressure resistance therefore are important factors in this process.
Polyamides and PEBA materials used in catheter production are typically based on the polyamide 12 (PA12) base structure. This polyamide is characterized by high strength and toughness, low water absorption and changes in properties associated therewith, and by good availability of the raw materials. PA12 is a common catheter material and, for reasons of good deformability, is popular as a base material for catheter balloons. The practical application for balloon components requires high pressure resistance, a low wall thickness, and high softness of the cones.
In order to improve the properties of the balloon when using a particular material, the orientation and crystallinity properties of the material are deliberately influenced. The elasticity, and hence the orientation, of the polymer can be improved by using additives, which increase the sliding qualities of the molecule chains against each other and/or reduce the crystallization of the polymer before deformation, for example by using suitable softeners and/or solvents.
PA12-based systems typically have a glass transition temperature of approximately 50° C. Temperatures above 50° C. are used to blow mold the balloon components. In order to impress a shape memory in these components, mold constraint and conditioning above or around the glass transition temperature are used, for example for folding and fixing the stent. In principle, heating the blow-molded components in the range of the glass transition temperature and above enables a relaxation of the stresses impressed by the plastic shaping. The relaxation causes hysteresis, for example, between the first and further, subsequent pressure stresses of the balloon component. As a competing effect, the crystallization of the polymer further progresses starting at 50° C.
Since polyamides during radiation sterilization typically suffer a loss of mechanical strength, and temperatures that are considerably higher than the glass transition temperature result in severe dimensional changes, EtO (ethylene oxide) sterilization has become established as a typical sterilization method for balloon catheters. The EtO sterilization processes are conducted under thermal conditions around 50° C. If the balloon catheter is EtO-sterilized, this thermal stress, in the presence of moisture, constitutes the absolute minimum of the relaxation of PA12-based components during the production of the catheter. In general, the balloon component is conditioned using thermal methods so as to obtain reproducible dimensions already after sterilization, which then no longer change considerably, even as a result of simulated aging and storage. However, this also means that the balloon compliance of the first inflation differs substantially from all subsequent inflation processes and is more drastic during the initial inflation. This effect is also associated with an increase in the diameter of the balloon component. Since the compliance of these components is determined during the first inflation, subsequent inflations of the balloon result in a certain systematic overdilation of the vessels. The increase in the diameter of the balloons, as the number of inflations and the inflation intensity increase, is therefore a safety-relevant quality criterion.
Less advantageous usage properties of PA12 balloons, which may be caused by harder balloon cones, for example, can be reduced, for example in terms of the material, by using a softer material—this being generally PEBA types or polymer mixtures of PA12 comprising such PEBA types. In general, however, the glass transition temperature of PA12 remains unchanged.
The viscoelastic properties of PA12 are even more pronounced with these PETA types. Starting at temperatures of approximately 50° C., these viscoelastic properties result in shrinkage, which leads to a distinct change of the balloon diameter between the first and any subsequent pressure inflations. In this way, this material optimization means that compromises are made in terms of the precision of the dilation behavior. The precision of the dilation is generally reduced as a result.